Method of testing transmission lines



Dec. 16, 1924. l,519,574

A. B. CLARK ET AL 'METHOD OF TESTING TRANSMISSION LINES Filed May 13, 1922 A TTORNEY Patented Dec. 16, 1924.

UNITED STATES PATENT OFFICE.

ALVA B. CLARK, OF BROOKLYN, AND VJILLIAM H. MARTIN, OF NEW YORK, N. Y., AS-

SIGNORS TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

METHOD OF TESTING TBANSLIISSION LINES.

Application filed May 13,

To @ZZ whom t may concern.'

Be `it known that we, ALvii B. CLARK and VILLIAM H. h/IARTTN, residing at Brooklyn and New York, in the counties of Kings and Bronx and State of New York, respectively, have invented certain Improvements in Methods of Testing Transmission Lines, of which the following is a specifica-tion.

This invention relates to a method of and leans for testing the transmission efficiency of lines and is particularly applicable to telephone systems in which the currents utilized for signaling purposes lie within the voice frequency range.

It has been the practice in former types of telephone and telegraph transmission systems to utilize different frequency levels for talking and for signaling; for example the signaling currents might have a frequency in the neighborhood `of 135 cycles, while the components of the voice current would have frequencies varying from about 300 to 2000 cycles. Interference between the different frequency levels was prevented by the use of selective apparatus. Such arrangements presented several. disadvantages in that they necessitated the provision at repeater and similar installations of separate repeating or relaying apparatus for the ringing and voice currents, for the reason that the coils and amplifiers elicient at voice frequencies were inefficient at the ringing frequency and vice versa. ln order to obviate such disadvantages and the ensuing duplication of equipment, arrangements have been proposed in the past for raising the level of the ringing or signaling channel to the same frequency level as that of the voice channel. Arran gements in which voice frequency current is utilized for signaling purposes are illustrated in the co-pending application, Serial No. 369,988, in the name of L. Espvenschied, and in the co-pending application, Serial No. 515,881, in the naines of A. B. Clarr, D. K. Gannett, and H. Nyquist, and reference may be` had thereto for a complete illustration of such systems.

ln accordance with this invention it is proposed to utilize the voice frequency signaling currents in systems of the above type as a means for testing the transmission etliciency of the circuits. Since the frequencies utilized for this type of signaling are in the important part of the voice range, and as 1922. Serial No. 560,634.

signaling currents are sent over the lines just prior to their employment for conversation, the eliiciency of transmission of the signaling current is a measure of the transmission efficiency of the circuit for the voice currents, which has obvious advantages. This feature would make possible the detecting of line or repeater troubles which lower the circuit transmission and thus avoid annoyance to customers who might otherwise be given a circuit when it was in uncommercial condition. An additional advantage would loe that no circuit time is required for testing purposes. In general the method of testing the circuit would consist in adjusting the ringing apparatus so that it would be operated near its margin, in which case au increase in equivalent of the circuit would cause failure of operation of the ringer and thus be detected. This might loe accoinplished by arbitrarily limiting the range of the ringer by adjustment of the sensitivity of the receiving circuit thereof to the maximum equivalent which shall be considered commercial for the given line. Circuit trouble which lowers the transmission efiiciencyr of the circuit beyond this commercial limit would then result in ringing failure and would cause the circuit to be reported in trouble. This method of testing the circuit might be' slightly modified by allowing the ringing apparatus to function at all times regardless of the line, but by adjusting a portion of the receiving apparatus of the ringer, such as a marginal relay, so that it would fail to function when the circuit equivalent increased beyond commercial. limits. The failure in functioning of this portion of the ringer might serve to report the unsatisfactory condition of the line. ln this arrangement it would be desirable that the amount of voice frequency signaling current sent out at the dista-nt end. be of a fixed or known value.

The invention may be more fully understood from the following description together with the accompanying drawing in the Figures l, 2 and 3 in which are shown arrangements embodying the features and principles of the invention. Fig. l illustrates schematically an arrangement of a transmission system in which voice frequency currents are utilized for signaling purposes. Fig. 2 shows the voice frequency ments 52 and 53.

signaling receiving and generating circuits suitable for this invention and adapted t'o be associated with the system shown in Fig. 1. The invention in a slightly In'odified forni may be illustrated by the addition` 'of the arrangements of Fig. 3 to those of Fig. 2 in manner to be pointed out hereinafter.

In Fig. 1 are shown two stations, A and B, interconnected 'by the line L. The transmission line L may include such apparatus as composite sets, phantom coils or repeaters illustrated schematically as the arrange- Bridged across the transmission line at the station A by conductors such as l and 21., 'or other suitable switching arrangements, are the circuits L, and L2: rlhe circuit Lj`is associated with a cycle generating apparatus 5E and a 20 cycle receiving apparatus 4. The circuit L2 is associ-ated with a voice frequency receiving apparatus 5 and a voice frequency generating apparatus 6. lVhen the operator at circuit A.

transmits low frequency signaling current into the line, such as 20 cycle current, a part of this current will be transmitted over circut L, to the 20 cycle. receiving apparatus 4 which, upon responding thereto, will operate the relay 8. The relay 8 will connect circuit L2 to the voice frequency generating apparatus 6, which in turn will transmit these signals over the line L by means of -current within the voice frequency range. The voice frequency signals lcoming in from a station such as B will be transmitted over circuit L, to the voice frequency receiving vapparatus 5, which, upon responding thereto, will operate the relay 7. The relay 7 will connect the circuit Ll to the 20 cycle generating apparatus 3 whereby ythe low frequency signaling currents suitable for 'operating the apparatus at the switchboard of station A will 'be transmitted thereto. Similar arrangements to those already described with respect to station A will be provided at station B associated with conductors 1 and 2 and the circuits L1 and L2.

ln Fig. 2 is shown in detail one arrangement of the low frequency and the' voice freipreucy receivers and generators associated with. the transmission line at each station. Associated with line L, is the filtering device F, which readily allows the transmission of the low vfrequency signaling currents,- sn'ch as 2O cycle currents, transmitted from the switchboard. This filtering device is so designed that it has a very high impedance to voice frequencies and therefore prevents the 20 cycle relays and circuits about to be described from causing an appreciable loss of energy to the speech currents or to the voice frequency signaling` currents. The lter also serves to prevent voice frequency harmonics in the 20 cycle-.current from being transmitted to L1 and thus to L2 and the voice frequency receiving circuit, and locking the latter in operated position. The line L1 is associated with the windings of a polarrelay 14, which is operated by thev20 cycle current transmitted Vfrom the switchboard. The 2O cycle current from the switchboard will be transmitted over the contacts of relay 9 and thence o ver conductors 13, 12 and 1() through the windings of the polar relay 14 and also through the winding of relay 11. This will operate the relay 11 as well as the relay 14. The operation 'of relay 11 closes the following circuit: from battery, conducto-r 29, contact ,and armature of relay 11, conductor 30, winding of relay 28, conductors 31 and 2() to ground. The closing of this circuit Will operate relay 28 which will disconnect conductor 26 from conductor 54 and connect it to conductor 18 thereby disconnecting the voi'cefre'quency receiving' apparatus which will be described later, from the line, and connecting the voice frequency generating' apparatus to the line.

rlhe forni of voice frequency generator illustrated 'in the dra-wing consists of an oscillating circuit controlled by the polar relay 14. The polar relay 1.4 in response to the 2O cycle signaling current alternately charges and discharges the condenser 16 in the oscillating `circuit `20 times a second. AlWhen the arinature of relay 14 rests on the right-hand Contact, the oscillating circuit includes the condenser 55, which lis charged to the potential of battery 56, conductor 21, the right-hand Contact and armature of relay 14, condenser 16, inductance 15, con-ductor 1S, make contact and armature of relay 28, conductor 2G, winding of transformer 19, conductors 17, 24 and 2O to ground. lVhen the armature of relay 14 rests on the left-hand contact, the circuit is the saine as above except that condenser 55 and conductors 21, 20 and 24 are no longer in circuit, the circuit being established directly from condenser 16 to conductor 17 through the left-hand contact. of relay 14. The induc't-ance-l,` together with capacity 16, is highly resonant at 90() cycles making the circuit oscillatory for that frequency. The alternate charging and discharging of the condenser 2O ltimes a second in the oscillating circuit creates a damped train of 800 cycle oscillations for each charge and for each disch-arge. These oscillations are transmitted from the. oscillating circuit through the transformer 19 to circuit L2 and thence out over the line. Therel'ation of the design of transformer 19 to the line impedance connected to line L, and to the impedance of the oscillating circuit just described, is such as to canse the line resistance to provide the proper damping coefficient to the oscillations to obtain the optinuun results. The

Ivoice frequency generator may be of any form, such as for example a vacuum tubo ll-ra oscillator or a motor generator instead of the form j ust described.

The voice frequency receiving arrangements are connected to line L2 through transformer 19 and include a series resonant circuit 32 tuned to approximately the signaling frequency, which may be, for example SOO cycles. The series resonant circuit 32 is of relatively low impedance as it is impracticable to construct such a circuit to have a very high impedance. To prevent this low impedance circuit from cansino' too great a loss in the telephone circuit the transformer 19, through which it is connected to the line is constructed to operate as an inequality ratio coil and to step up the impedance in the proper proportion. ln practice the series resonant circuit may not be tuned to the exact carrier frequency of vthe incoming signal, such as SOO cycles, but is tuned slightly lower. This results in a beat phenomenon which causes the effect that the demodulated wave produced by the detecting tube 34 has greater amplitude than in the case when the circuitis tuned exactly to the incoming frequency. This feature results in greater efficiency of response to the signal. Associated with the series resonant circuit is a detector 34 of the well known vacuum bulb type. Associated with the output transformer 36 of the tube is a bridged condenser which tunes it to be resonant at 40 cycles, which is the frequency of occurrence of the trains of S00 cycle oscillations transmitted over the line L from the generating circuit at the distant station. This arrangement results in maximum sensitivity to the ringing signal and low sensitivity to interference. The 40 cycle current thus produced by the detecting action of the T"acnuin bulb 34 is transmitted by the tuned output transformer 36 to a polar relay 37 and serves to operate this relay at the saine frequency of 40 cycles. This vibrating operation of the polar relay 37 causes sustained operation of the succeeding neutral relay 40 in the following` manner: Each time the armature of the polar relay 3"( strikes the lower contact, condenser 38 is charged to the potential of battery 56 through the circuit from condenser 55 (which is maintained at the potential of battery 56, as has been pointed out) over conductor 45 and 44 the lower relay contact and the relay armature. The other terminal of condenser 38 is grounded through conductors 39, 43 and 20. When the armature of the polar relay 37 strikes the upper contact condenser 38 discharges through relay 40 and conductors 4l, 42, 43 and 2O to ground. The sensitivity of relay 40 is such that when the polar relay operates with a speed of approximately 40 cycles the discharges of condenser 3S produce enough current to opuate said relay. peration of the polar relay at an appreciably lower speed than 4() cycles, however, does not result in sufiicient current in relay 40 to cause its operation. This prevents false operation of relay 40 from interference of such a nature as to cause a low frequency operation of the polar relay.

Then relay 4()` operates, its contacts open up the circuit which otherwise exists through the winding of relay 46. This circuit is as follows: from ground and battery 56, conductor 23, coil 22, conductor 45, winding of relay 46, resistances 50, contact of relay 40 and conductors 42, and 20 to ground. Upon opening of the contact of relay 40, however, the current does not iinmediately cease flowing in the winding of relay 46 but continues to flow into condenser 57 until the latter .is charged to the potential of the battery a6. This causes relay 46 to release with a certain time lag with respect to the operation of relay 40, which lag is determined by the sensitivity of relay 46 and by the time constant of the condenser 57 and the resistance 50 which is in series with relay 46. This time lag serves as an additional check to prevent any false operation of the signaling means which might. occur should the actual voice currents which are not intended for signaling purposes serve to actuate the relay 37 and the relay 40. its any such actuation of these relays would only be of a temporary nature, the relay 40 would be restored to normal before the relay 46 could release under these conditions, discharging condenser 57 and reestablishing the current through the latter relay. When the relay 46 is released, the following circuit is closed: from ground, conductors 20, 43 and 42, armature and contact of relay 46, conductor 47, winding of relay 9 to battery and ground, thereby operating relay 9. This will connect to the line L1 the conductors 48 which lead to a source of suitable low frequency ringing current such as 2O cycle current to operate the switchboard apparatus. itt the close of the ringing signal, polar relay 37y stops vibrating` and relay 40 is released, returning to its normal unoperated position. Relay 46, however, operates with a time lag which is shorter than the lag at the beginning of the ring, and which is determined by the time constant of the condenser 56 and its discharge path through the two resistances 50 in parallel. This latter feature has been found valuable in preventing interference with the ringing signal by disturbing currents.

As has been pointed out heretofore, the method of testing the transmission efficiency ofthe line circuit consists in general in adjusting the ringing circuit and apparatus so that it will be operated near its margin in which case an increase in the transmission equivalent of the circuit beyond that delll) I so that it siraxble lor commercial purposes. would cause failure of Aoperation of the ringer and would thus be detected. j lThis might be 'accomplished by adjustment ot the relay 3T. The relay 37 in accordance With this ini'fenti'on Will be adjusted so tlratwhen fthe line L is at or above the maximum transmission equivalent Idesirable lor commercial purposes, the voice frequency signaling vcurrent Will not operate it. The relay Si? however will be opeinted l y the voice frequency signalii-ifg `currents when the line la is hele-W said inaxiniuin equivalent. Accordingly any circuit t'roifl.v l'le which might lower the transmission fellici-ency ot the circuit, or in other Words raise Ithe equivalent bey-ond the cor-nnier'cial limit would res-ult in ringing failure and cause the circuit to be reported in trouble.

The invention :might be slightly modi'licd e 'actual ringing signal would not ail to inflection upon 'an increase in line equivalent, but that another portion 'ot the ringing receiving apparatus would 'fail to vfunction under lsufch conditions and thus give 1an indication ol the circuit condition. Such an arrangement would 'be illustrated by combi-ning Flag. -3 with Fig. 2 by bridginfg conductors: 60 and 6l across conductors 460 and 6l. Associated with conductors 60 land 6l" is the Winding of a relay 37', similar to relay 37 oi Fig. i2 except for "a slight difference in adjustment which will be pointe-d out later. Associated YWith and controlled by the armature 'ot relay 37 is ia rel-ay 405. The relay 40 in turln 4controls the relay 46', a time larg` being introduced between the operation lof these relays which is somewhat less than that introduced bet-Ween relays 40 and 46 ias described above. The circuit arrangement 'oie relays 40 and 46 is substanti the same as that with respect to relays 40 and 46 oft Fig. 2, lilre reference characters being used to denote like parts in both figures and no detailed description thereol will ibeigii'ven. In this modilication 4of the invention the relay 3? will be adjusted so that the n'alin'g current Will operate it under pracally @all line condltions and accordin'glyI `there will be at no linie a tai-lure in the ringing signal itself. The relay y37 lroivever will vbe so adj usted that it will be ope-rated by the signaling current Whenever the line isv -beloiy the maximum equivalent suitable 'for 'commercial purposes but it will not be operated by the A aling :current when the `line is at oir above this maximum equivalent. With this.arrangement, it the lineiequivalent is satisfactory troni a commercial standpoint, the voice lrequency signaling current transmitted lover the line Will operate kboth relay 87 and relay 37. In a manner previously Ipointed out relay 3Q' Will cause relay 46 to release, which will complete a circuit lor the ringing signal over conductor 47. The 'operati-on of relay 37 will in like manner cause the release ol relay v46 which will hold 'open at its contact the following -alarm 'circuit which would otherwise be Icompleted Iover the second armature `and contact `of relay '46; from ground, conductor 43', 4|arma-ture and contact ol relay 46", conductor 62, contact and armature 63 'of relay 46, conductor 64, ylilain'ent olI lamp `65, to battery 66, and ground. Accordingly il? the line equivalent is satisfactory 'both relays and 37 will be operated, the actual ringing signal Will function, `and the alarm lamp will not operate. However il' due to circuit trouble the transmission equivalent lof the line n'oyv increases above the maxiinufm limit suitable for 'commercial purposes, the voice frequency signalilng current Will operate the relay 37 but Will not oper-ate the relay 87. Under these 4conditions the 'circuit for alarm lamp 65 will be closed at the contact el .relay 46 will not be opened at the Contact yof relay 46 'and the alarm lsignal 65 `Will 'operate to """ve -an indication 'of the unsatisfactory 'conc tion ot the line.

ll/'hi-le the arrangements 'ol the invention have been illustrated Pas Ienrborhed in certain specific lar-rangements which are deemed desirable and with a certain type of voice t-requency signaling circuit, it is understood that they 'are capable ot embodiment in other forms, arrangements, and "circuits Without departing from the spirit nl' the intention as 'defined 'in the appended claims.

That is claimed is: y

l. In a telephone transmission system in which the 'currents utilized for signaling purposes lie within the icentral portion ot the voi-ce frequency range, 'the method ot testing the trans-mission ellicielncy of the .line for talking purposes which consists in adjusting fthe signal receiving apparatus so that it "iv-ill operate 'only Wli'en the transmission `equivalent of the line is Within certain limits.

2. In a telephone transmission system n which Athe currents utili-Zed for signali purposes lie Within the central portion ot' the voice frequency nan-ge, the method of testing the transmission efficiencyf of the line which consists 1in fapl'aly-ing Asai-d voice ltreqnency signalling cur-rents to one end oi `'the line lto eiiiect a responsive device tat the other end of :said ne, and a ting :said responsive device so that it will operate only when the cur-rent received thereat is equal to that received when the line circuit is at 'its lowest desirable zetliciency for talking punposes.

3. ln ya telephone transmission system in which the lci'irrents util-ized for signaling purposes lie Within the central portion el' the voice frequency mange, the method ot testing the transmission eli-ioiency ot the line for -talking purposes which consists in -a-dyroo lli)

justing a portion of the signal receivingI apparatus so that it will fail to operate when the transmission equivalent of the line not Within certain limits.

4. A telephone transmission system comprising a line. means tor talking over said line, means i'or applying currents Within the central `portion ot the voice frequency range to one end ot said line for purposes of signaling) and a signal receiving apparatus at the other end of said line responsive to said voice frequency signaling currents, said receiving apparatus being adjusted to respond to said voice frequency signaling currents only when the transmission equivalent to the line is Within certain limits.

5. A telephone transmission system comprising a line, means for talking over said line, means for applying currents Within l the central portion oi? the voice frequency range to one end of said line for purposes ot signaling, a signal receiving` apparatus at the other end ot said line responsive only to said voice frequency signaling currents, a relay in said receiving apparatus, a signal circuit controlled by said relay, a second relay in said receiving apparatus, and an alarm circuit controlled by both of said relays said second relay being adjusted so that it will operate only when the line equivalent is within certain limits and said irst relay being adjusted so that it normally will be operated When the line equivalent is beyond said limits.

In testimony whereof, We have signed our naines to this specilication this 11th day of May 1922.

ALVA B. CLARK. WILLIAM H. MARTIN. 

